Arrangements embodying pick-up tubes



Feb. 24, 1959 D. e. PERKINS ARRANGEMENTS EMBODYI NG PICKUP TUBES Filed July 19, 1955 Ail-PI ER? FIG.5.

PIC-3.4.

ARRANGEMENTS EMBODYING PICK-UP TUBES Denis Gordon Perkins, Gerrards Cross, England, assignor to Electric 85 Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain 7 Application July 19, 1955, Serial No. 522,987

Claims priority, application Great Britain July 20, 1954 12 Claims. (Cl. 315-11 This'invention relates to arrangements employing pickup tubes such as are employed for the generation of signals suitable for use in television transmitting systems.

In one such arrangement it has been proposed to employ a pick-up tube having a target of the photo-electrically emissive charge storage type and after charges are set up on this target as a result of light images projected thereon it is scanned in a line-by-line manner by a light spot so as to generate signals representative of said light images. The charged elements of the target are scanned with a low velocity electron beam simultaneously whilst being scanned by the light spot so as to stabilise the elements of the target electrode to a potential at or near the potential of the cathode which generates the electron beam.

In such arrangement the signal electrode of the target electrode is connected to a load resistance and during operation two signals are generated in said load resistance, one corresponding to the rise in potential during scanning by the light spot and the other corresponding to the fall in potential when the electron beam stabilises the elements of the target electrode to cathode potential. The former is the required signal and the latter is an'unwanted signal and although in theory the unwanted signal is a steady signal, it does in practice provide a spurious background to the wanted signal.

The object of the present invention is to provide 'an improved arrangement in which a target electrode is scanned simultaneously by a light spot and by-a cathode ray beam whereby separation of the unwanted and wanted signals is eifected.

According to the present invention there is provided a pick-up tube arrangement comprising a pick-up tube having a photo-electrically emissive charge storage target electrode which can acquire charges representing a light image, a cathode for generating an electron beam and means for scanning said electron beam over the surface of said target electrode to stabilise the target electrode substantially to the potential of said cathode which generates the scanning beam, means for scanning said photoemissive target electrode with a light spot simultaneously with the scanning of the target electrode by said electron beam to release photo-electrons from the target electrode depending on the charges stored on said target electrode, it being arranged that the position on the target of the scanning electron beam is displaced from the position on the target of said scanning light spot thereby to cause photo-electrons released during light spot scanning to be separated from electrons of the scanning electron beam not required for stabilising the target electrode and means for separately collecting substantially only photoelectrons released during light spot scanning and means for generating signals dependent on the charges acquired by said target electrode from the photo-electrons so collected by said collecting means.

In order that the said invention may be clearly understood and readily carried into etfect, it will now be more 'fully described with reference to the accompanying drawings, in which: a a

Figure lie a diagrammatic view of a pick-up tube arrangement in accordance with one embodiment of the invention,

Figure 2 is a view of a portion of another kind of pickup tube suitable for use in the invention, and

Figures 3, 4, 5, 6 and 7 respectively illustrate arrangements for improving the separation between the electron beam and the photo-electrons. I

As shown in Figure 1, an arrangement includes a pickup tube comprising an envelope 1 having an end window 2 adjacent to which is a photo-electrically emissive charge storage target electrode comprising a transparent conducting electrode 2a, an insulating transparent sheet 2b having on its surface opposite to the electrode 2a a multiplicity of photo-electrically emissive mosaic elements 20. The electrode 2c may be earthed and in front of the elements 2a a mesh electrode 2d is provided maintained in operation at a positive potential of about 5 volts with respect to the potential of the cathode 8 hereafter referred to. The target electrode is adapted to receive light images through an optical system indicated at 3, the images being projected through a halt silvered mirror 4 and through the window 2 on to the elements 2c. Photo electrons are thereby emitted from the elements Zc'and are mainly collected by an anode 5 arranged internally of the envelope 1, which; electrode may be maintained in operation at, for example, a positive potential of 200 volts with respect to the cathode 8. As a result of the emission of photo electrons the elements of the target electrode acquire positive potentials. The target electrode is arranged to be scanned by a flying light spot which is generated by a cathode ray tube 6, the light spot from which is projected via an opticalsystem 7 on to the half-silvered mirror 4 from which it is projected on to the elements 2c, the light spot from the cathode ray tube 6 scanning the target electrode simultaneously with the projection of the light image thereon. Scanning with the light spot causes the emission of possible further photo electrons, resulting in a-further increase in the positive charges developed on the target electrode, the charges being limited by the potential of the electrode 2d. Photo-electrons emitted from an element 2c due to the light spot are able to pass the electrode 2d until the element reaches the potential of. the electrode 2d, these photo-electrons passing the electrode 2d serving to produce the wanted signals.

The released photo-electrons due to the light spot passing the electrode 2d depends on the potentials acquired by the elements 20 as a result of the light image and hence the. photo-electrons released by the light spot and'passing the electrode 2d constitute a beam modulated in accordance with the positive charges acquired by the charge image and which serves to generate the wanted signals as hereinafter described. The target electrode is also arranged to be scanned by an electron beam which serves to restore the elements of the target electrode to an equilibrium potential which corresponds substantially to the potential of the cathode of an electron gun provided in said pickup tube and which generates said scanning beam. The electron gun comprises a cathode 8, a cathode screen 9 and a limiter electrode 10, these electrodes being maintained in operation, for example, at zero volts, at 20 volts and at +300 volts, respectively. The electron beam from the cathode 8 is caused to scan the elements 20 on the side thereof opposite to that on which the light image is projected in an orthogonal manner, for which purpose the pick-up tube 1 is immersed in a longitudinal magnetic field generated by a solenoid coil 11 surrounding said pick-up tube. Scanning of the electron beam from the cathode 8 is efiected by the provision of scanning coils 12 which serve to deflect the electron beam in mutually co-ordinate directions, scanning of thetarget electrode by the electron beam and by the light spot being elfected in synchronism for which purpose a suitable scanning generator 13 is pfovided which feeds scanning currents not only to the coils 12 but also to the deflecting coils 14 of the cathode ray tube 6 which generates the flying light spot. During scanning of the elements 20 by the electron beam, electrons in the scanning beam which are not accepted by the targetelectrode arereturned to the electron gun where they are collected by the limiter electrode 10. The path of electron beam generated by the cathode 8 is indicated diagrammatically by the full-line lS and the returned electrons by the dotted-line 16. Photo electrons released during scanning by-the light spot which are indicated by the chain dotted line 17 will also, due to the presence of the longitudinal magnetic field set up by the solenoid 11, be directed towards theelectron gun. It is arranged that the light spot from the cathode ray tube 6 scans the target electrode some lines in advance of the electron beam 15 so that the photo-electrons, released during the light spot scanning will be physically displaced from the returned electrons 16 of the scanning electron beam both beams being maintained focussed by the field of the solenoid 11. Thephoto-electrons 17 are collected by a collecting electrode "18 disposed adjacent to the electron gun and the returned electrons 16 by the electrode and since the electrons in the scanning beam and the photo electrons 17 are physically separated and are collected by separate electrodes, so the wanted and unwanted signals areeffectively separated. The relative displacement of the returned electron beam 15 and the photoelectron beam 17' may correspond in time to slightly less than the field black-out period, i. e. a period correspondfirig approximately to ten scanning lines. The collecting electrode 18 may be connected to a load resistor across which the wanted signals are set up but preferably the collectingelectrode 18 constitutes the first dynode of an electron'multiplier, the other dynodes of which areindicated at 19 whereby the photo-electrons can be amplified in the electron multiplier to bring them well above the noise level of the first valve of the amplifier usually employed for amplifying the generated signals. As shown the last electrode of the multiplier is connected to a load resistor 20,

If desired the pick-up tube shown in Figure 1 may incorporate'a stage of image amplification and an example ofsiich an arrangement is shown in Figure 2. In such an arrangement the light image is projected through the optical system 3'on to a photo-electric cathode 21 which releases a photo-e1ectron image which is accelerated on to a target enctrade which comprises a leaky dielectric sheet 22 andphoto-electrically emissive mosaic elements 2c. Thelectr'onsfrom the photo-cathode 21 are focussed by the 'fsol'enoid coil ll and are accelerated on to the sheet 22 by a ring electrode 23 which is maintained at' a suitable accelerating potential. The photo-electrons are thereby caused 'to impinge on the sheet 22 with such a high velocity that they cause the release of a large number of secondary electrons so that the sheet 22 becomes positively charged. This positive charge leaks through the sheet 22 so that the photoelectric elements 2c which are provided on thes'ur'face of the sheet 22 opposite to that on whichthe electron image is projected likewise acquire positive charges. The elements 2c are then scanned in;a ma nner similar to that described with reference to Figure l, the cathode ray tube for generating the flying light spot being arranged on the side of the elements 2c remote from the cathode 21 as shown.

If desired as an additional step to ensure that the returned electrons 16 of the scanning electron beam are not intercepted by said collecting electrode 18 for the photo-electrons, a magnetic field or electrostatic field may be set up in front of the electron gun so as to cause the photo-electrons and the unaccepted electrons of the 4 electron scanning beam to diverge so as to enable the spacing between the collector electrode 18 and the electron gun to be increased.

Figure 3 of the drawings illustrates an example of a magnetic field suitable for the purpose mentioned in the preceding paragraph. In the example shown in Figure 3, instead of the solenoid 11 embracing the electron gun it terminates a short distance in front of the electron gun so that the magnetic field in the vicinity of the electron gun is diverging. The electrons and the photo-electrons will likewise follow these diverging paths so that the electron gun which is indicated generally at 24 and the collecting electrode 18 can be more widely separated, as indicated.

Figure 4 illustrates a further example in which an additional solenoid coil 25 is employed which sets up a weaker field compared with thefield set up by the coil 11, the electron gun 24 and the collecting electrode 18 being immersed in the weaker field of the coil 25. With such an arrangement a divergence of the electrons and photo-electrons will occur so that the electron gun 24 aridthe collecting electrode 18 can be more widely. spaced.

Figures 5 and 6 illustrate a further example or magne'ticmeans for achieving the same purpose.

As 'shown in Figure 5 the returned electron beam 16 is caused to be more widely separated from the photo-electron beam 17 in the vicinity of the electron gun by the provision of three polepieces 26, 27 and 28, which aremagnetized so that the two outer electrodes 26 and28 are northpoles and the intermediate electrode 27 is a south pole. This magnetisation may be accomplished as shown in Figure 6, which is a side elevation of the arrangement shown in Figure 5 bymagnets 29 and 30 arranged outside of the envelope 1 and having pole pieces 31, 32 and 33 which serve to magnetise the pole pieces 26, -27 and 28 with the polarity shown, the latter having portions 34, 35 and 36 which extend into close proximity to the pole pieces 31, 32 and 33.

Figure 7 of the drawings illustrates an example of electrostatic means for elfecting a wider separation of the electron beam and the photo-electron beam in which two pairs of lift-plates 37 and 38 are employed, the lift-plates 37 being maintained at suitable negative and positive potentials as indicated so that the lift-plates cause, in conjunction with the field of the solenoid 11, relative displacement of the electron beam and the photoelectron beam in the vicinity of the electron gun 24 and the collecting electrode 18, so that these electrodes, as in the previous examples shown in Figures 3 to 6, can be-more widely separated.

In order to obtain a good signal-to-noise ratio and to avoid spurious signals itis desirable that the-area oi the collecting electrode for the photo-electrons or the first dynode as the case may be is maintained small so that the number of photo-electrons other than those generated by the light spot which are intercepted by said electrode are kept at a minimum. For this purpose said collecting electrode or first dynode is preferably disposed at the position of a focal point in the electron beam 17 produced by the scanning light spot. It is desirable that the electron scanning beam 16 is not finely focussed on the target electrode and hence the collecting electrode or first dynode for the photo-electrons may be displaced along the tube relatively to the limiter electrode10 of the electron gun on which the returned electrons from the electron scanning beam are collected.

What I claim is:

1. Apick-up tube arrangement comprising a photocmissivetarget electrode, means for producing a scanning light spot, means for producing a scanning electron beam, means comprising a half-silvered mirror for projecting a light image onto said target electrode, means for producing a longitudinal magnetic field to cause the electron beam to scan said target electrode in an orthogonal manner, and a collecting electrode tocollect photo-electrons released by said light spot, it being arranged that the light image and the flying light spot are directed on to the same side of said target electrode via said half-silvered mirror, and the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, said longitudinal magnetic field thereby causing photo-electrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam and said collecting electrode is positioned to collect substantially only those electrons released due to scanning by said light spot.

2. A pick-up tube arrangement comprising a photoemissive target electrode, means for producing a scanning light spot, means for producing a scanning electron beam, means comprising a half-silvered mirror for projecting a light image onto said target electrode, means for producing a longitudinal magnetic field to cause the electron beam to scan said target electrode in an orthogonal manner, and an electron multiplier having a plurality of'dynodes, one dynode of which is also a collecting electrode for collecting photo-electrons released by said light spot, it being arranged that the light image and the flying light spot are directed onto the same side of said target electrode via said half-silvered mirror, and the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, said longitudinal magnetic field thereby causing photo-electrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect, and said electron multiplier to multiply substantially only those electrons released due to scanning by said light spot.

3. A pick-up tube arrangement comprising a photo emissive cathode, means for projecting a light image on to said cathode, means for accelerating photo-electrons released from said photo-emissive cathode, a target electrode comprising a sheet of leaky dielectric and photoemissive elements provided on the side of said sheet renote from said cathode, means for producing a scanning light spot, means for producing a scanning electron beam, means for producing a longitudinal magnetic field to cause the electron beam to scan the target elec trode in an orthogonal manner, and a collecting electrode to collect photo-electrons released by said light spot, it being arranged that photo-electrons released from said photo-emissive cathode are accelerated onto said target electrode so as to set up positive charges on said sheet of leaky dielectric which charges leak to said photo-emissive elements, and the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, said longitudinal magnetic field thereby causing photo-electrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect substantially only those electrons released due to scanning by said light spot. I

4. A pick-up tube arrangement comprising a photoemissivecathode', means for projecting a light image on to said cathode, means for accelerating photo-electrons released from said photo-emissive cathode, a target electrode comprising a sheet of leaky dielectric and photoemissive elements provided on the side of said sheet remote from said cathode, means for producing a scanning light spot, means for producing a scanning electron beam, means for producing a longitudinal magnetic field to causethe electron beam to scan the target electrodein an orthogonal manner, and an electron multiplier having a plurality of dynodes, one of which is also a collecting electrode for collecting photo-electrons released by said light spot, it being arranged that photo-electrons released from said photo-emissive cathode are accelerated on to a sras'ii said target electrode so as to set up positive charges on said sheet of leaky dielectric which charges leak to said photo-emissive elements, and the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, said longitudinal magnetic field thereby causing photo-electrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect, and said electron multiplier to multiply substantially only those electrons released due to scanning by said light spot.

5. A pick-up tube arrangement comprising a photoemissive target electrode means for producing a scanning light spot, an electron gun for producing an electron beam, means for producing a longitudinal magnetic field to cause the electron beam to scan said target electrode in an orthogonal manner, a collecting electrode to collect photoelectrons released by said light spot, and means for producing at least one of a magnetic field and an electrostatic field to cause divergence of electron beams in the vicinity of said electron gun, it being arranged that the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, the longitudinal magnetic field and the diverging 'field thereby causing photo-electrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect substantially only those electrons released due to scanning by said light spot.

6. A pick-up tube arrangement comprising a photoemissive target electrode, means for producing ascanning light spot, an electron gun for producing an electron beam, means for producing a longitudinal magnetic field to cause the electron beam to scan said target electrode in an orthogonal manner, an electron multiplier having a plurality of dynodes one of which is also a collecting electrode for collecting photo-electrons released by said light spot, and means for providing at least one of a magnetic field and an electrostatic field to cause divergence of electron beams in the vicinity of the electron gun, it being 7 arranged that the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, the presence of said longitudinal magnetic field and the diverging field thereby causing photoelectrons released due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect, and said electron multiplier to multiply substantially only those electrons released due to scanning by said light spot.

7. A pick-up tube arrangement comprising a photoemissive target electrode, means for producing a scanning light spot, an electron gun for producing an electron beam, means comprising a half silvered mirror for projecting a light image onto said target electrode, means for producing a longitudinal magnetic field to cause the electron beam to scan said target electrode in an orthogonal manner, a collecting electrode to collectphoto-electrons released by said light spot, and means for producing at least one of a magnetic field and an electrostatic field to cause divergence of electron beams in the vicinity of said electron gun, it being arranged that the light image and the flying light spot are directed on to the same side of said target electrode via said half-silvered mirror, and the scanning by the electron beam of the target electrode is displaced by a number of scanning lines from the light spot, said longitudinal magnetic field and said diverging field thereby causing photo-electrons re-v leased due to the light spot scanning to be substantially separated from the electrons returned from said target electrode due to scanning with the electron beam and said 7 only those electrons released due to scanning by said light spot.

1A ipicloup 'tube .arrangement comprising :a photoemrssive target 'electrode,means for producing a. scanning light spot,-an electronrgun'for producing anelectron beam, 1

means. comprising a half-silvered mirror foriprojecting a light imageon to said target electrode, means for producingia longitudinal magnetic fieldto cause the electron beam tolscan said target electrode. in anorthogonal mannor, an electron multiplier having aplurality of dynodes one of which is also a collectingelectrode for collecting photo-electrons released by said light spot, and means for producing at least oneof a magnetic field and an electrostaticfield to cause divergence of electron beams in the vicinity of said'electron gun, it being arranged that the light image andthefiying light spot are directed on to the same side .ofi'said targettelcctrode'viasaid half-silvered mirror, and the scanning by'the electron beam -of-the target electrode is'displacedby'a number of scanning lines fromathe light spot, said longitudinal magnetic field and said diverging field thereby causing .photo electrons released due to the light spot: scanning to be'substantially separated from the electrons returned from said target electrode due to scanning with the electron beam and said collecting electrode is positioned to collect,and said electron multiplier to multiply, substantially .only those electrons released due to scanning'bysaid light spot.

'9. A pick-up tube arrangement comprising a photoemissive cathode, means for projecting a light image on to said cathode, means for accelerating photo-electrons released. from said photo-emissivecathode, a target electrode comprising a sheet of leaky dielectricand photo emissive elements provided on the side of said sheet remote from said cathode, means for producing a scanning light spot, an electron gun for producing an electron beam, means for producing a longitudinal magnetic field to cause the electron beam to scan the target electrode in an orthogonal manner, a collecting'electrode to collect photo-electrons released by said light spot, and means for producing at least one of a magnetic field and an electrostatic field to cause divergence of electron beams in the vicinity of said electron gun, it being arranged that photo-electrons released from said photo-emissive cathode are accelerated on to said target electrode so as to set up positive charges on said sheet of leaky dielectric which charges leak to said photo-emissive elements, and the scanning by the electron beam of the target electrode is displaced by'a number of scanning lines from the light spot, said longitudinal magneticfield and said diverging field thereby causing photo electrons releaseddue to the light spot scanning to be substantially separated from the electrons returnedfrom said target electrode due to scanning with the electron beam, and said collecting electrode is positioned to collect substantially only those electrons released due to scanning'by said light spot.

10. A pick-up tube arrangement comprising a photoemissive cathode, means for projecting a light image on to said cathode, means for. accelerating photo-electrons released from said photo-emissive cathode, a target electrode comprising a sheet of leaky dielectric and photo emissive elements provided on the side of said sheet remote from said cathode, meansfor producing a scanning light spot, an electrongun for producing an electron beam, means for producing a longitudinal magnetic field to cause the electron beam to scan the target electrode in'a'n orthogonal manner, an electron multiplier having a plurality of dynodes one of which is also a collecting electrode for collecting photo-electrons released by said light spot, and means for producing at least one of a magnetic field and an electrostatic field such as to cause divergence of electron beams in the vicinity of said electron gumit being arranged that photo-electrons released from said photo-emissive cathode are accelerated on to saidtarget electrode. so. asl'to wse't uppositivelcharges on said sheet of 1 leaky T dielectricwhichscharges *lealci to. said photo emissive *elemerita'pandr the: scanning 2 :by the telectron beam :of said; target electrode is. displaced byl a number softscanning lines from the; lightaspot; saids'lo'ngitu- 'dinalsmagnetic field. and? said 'divergingifield thereby causingcphoto-electrons released duc to. the light=spot scanning to :be substantially separated fromthe-electronS returned from i said target electrode due to scanning :with; the electrou beam, and said.collectiugrelectrode is positioned-to photo emissive'target electrode with alight spot simulltaneouslyuith' the scanning of the target electrode by said electron "beamto .release photoelectrons from the target electrodedepending on the charges stored on said target electrode, it being arranged that the position on the target of the scanning electronbeamis displaced from the position on the targetof said scanning light spot thereby to cause .photoeelectrons released during ,light spot scanning tobe. separated. from electrons o'tthe scanning electron beam not required for stabilising the target electrodeand means for separately collecting substantially only photo-electrons released during light spot scanning and means, for generating signals depe'ndent on the charges acquired by saidtarget electrode from the photoelec'trons socollected by said collecting means.

12. A pick-up tube arrangement comprising a pick-up tube having aphoto-electrically emissive charge storage targetelectrode which can acquire.chargesrepresenting a lightimage, a cathode for generating an electron beam and means for. scanning said electron beam in the. presence of a longitudinal magnetic field over the surface of said target electrode to stabilise the target electrode substantially'to the potential of said cathode which generates the scanning beam, means for scanning said photoemissive target electrode with alight spot simultaneously with thescanning of the target electrode by said electron beam to release photo-electrons from the target electrode depending on the charges stored on said target electrode, it being arranged that the position on the target of the scanning electron beam is displaced from the position on the target of said scanning light spot thereby to cause photo-electrons released during light spot scanning tobe separated from electrons of the scanning electron beam not requiredfor stabilising tbetarget electrode and maintained separated by-said field and means for separately collecting substantially only photo-electrons released during light-spot scanning and means for generating'signals dependent on the charges acquiredby said target electrode from the photo-electrons so collected by said collecting means.

References Cited in the file ofthis patent 

